I feel like all this talk about payloads is distracting from what a huge challenge the EDL is going to be, which is the primary objective in the first place.

True. The core is the supersonic retro propulsion attempt. It seems everyone of NASA's large scale Mars plans needs it, as does SX's.

I wonder how many people realize just how low Mars atmospheric pressure is. It's about 1/160 of Earths at the surface but at 80000 ft it's down to 0.08% of Earth SL, which is about 1/3 at the same height in Earth's atmosphere.

the next question of course is how much of that velocity can they cancel with thrust alone?

Thinking about it I'm guessing they are leaving engine start to late in the entry and relying on a fairly conventional initial stage, with PICX taking most of the brunt.

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I feel like all this talk about payloads is distracting from what a huge challenge the EDL is going to be, which is the primary objective in the first place.

True. The core is the supersonic retro propulsion attempt. It seems everyone of NASA's large scale Mars plans needs it, as does SX's.

I wonder how many people realize just how low Mars atmospheric pressure is. It's about 1/160 of Earths at the surface but at 80000 ft it's down to 0.08% of Earth SL, which is about 1/3 at the same height in Earth's atmosphere.

the next question of course is how much of that velocity can they cancel with thrust alone?

Thinking about it I'm guessing they are leaving engine start to late in the entry and relying on a fairly conventional initial stage, with PICX taking most of the brunt.

There was a paper linked in another thread suggesting that supersonic retropropolsion can actually add to balistic drag (in addition to anything the rockets do by being rockets) by being sloped high-pressure zones funneling the oncoming air onto the heat shield, compressing the air the capsule is actually going through. The Superdracos are set up for that sort of approach, though not as efficently as if they were equadistant around the Dragon II perimeter.

Certainly. But that isn't a reason to rule out SpaceX trying to put on some payload on this initial mission. Water ISRU is a lynchpin of this whole architecture, is low TRL, and so trying it as soon as possible is likely a priority for SpaceX (after demonstrating EDL, of course).

Except the usable-supply-of-water-on-Mars part, that is. This is still a non-trivial problem to solve. Of course, SpaceX might "cheat" and send a few liters of water to Mars along with a sub-scale Sabatier reactor to process some Martian CO2 (and not coincidentally suck all the battery life from the Red Dragon, and then some).

Certainly. But that isn't a reason to rule out SpaceX trying to put on some payload on this initial mission. Water ISRU is a lynchpin of this whole architecture, is low TRL, and so trying it as soon as possible is likely a priority for SpaceX (after demonstrating EDL, of course).

Except the usable-supply-of-water-on-Mars part, that is. This is still a non-trivial problem to solve. Of course, SpaceX might "cheat" and send a few liters of water to Mars along with a sub-scale Sabatier reactor to process some Martian CO2 (and not coincidentally suck all the battery life from the Red Dragon, and then some).

Frankly, carrying ordinary-old water in a jug might be the best way to bring the hydrogen to Mars to start up the Sabatier stuff without the burdens of LH2 storage. Just electrolize the stuff after landing, with the O coming in handy for LOX and O2 for astronauts later.

I wonder how many people realize just how low Mars atmospheric pressure is. It's about 1/160 of Earths at the surface but at 80000 ft it's down to 0.08% of Earth SL, which is about 1/3 at the same height in Earth's atmosphere....

It's still enormous (logarithmically/relatively speaking) compared to the vacuum of space.

Also, 160th /pressure/ at the datum, but at the lower landing sites being considered, it's more like 1/100th, and given that CO2 is denser at the same pressure than Earth's air and the fact that Mars is usually around 215K, it's actually closer to 1/50th the /density/ of Earth standard pressure. If you're calculating things like ballistic coefficient, it's actually density that matters most, not pressure.

But anyway, it doesn't completely negate your point, but that's about a factor of 3 better than you suggest.

By the way, on Earth, it's very rare for any land to be below sea level, but about half of Mars is below "datum," and some parts of Mars (Hellas Basin) actually have about twice the pressure as datum... so I don't really think it's fair to pick "datum" as analogous to "sea level"... I think early landing sites and early habitation will be more like -5km, which has almost 60% higher pressure/density than datum.

Chris Whoever loves correction loves knowledge, but he who hates reproof is stupid.

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Certainly. But that isn't a reason to rule out SpaceX trying to put on some payload on this initial mission. Water ISRU is a lynchpin of this whole architecture, is low TRL, and so trying it as soon as possible is likely a priority for SpaceX (after demonstrating EDL, of course).

Except the usable-supply-of-water-on-Mars part, that is. This is still a non-trivial problem to solve. ...

Water is, in fact, ubiquitous on Mars. It exists over the whole planet in the atmosphere and in the regolith. In levels high enough that, with enough work, you can get usable amounts of it. And this is exactly the sort of thing you'd want to demo as soon as possible.

Just doing some electrolysis and sabatier on the surface would be nearly pointless. It's well enough understood and even used today on ISS. The point of a demo isn't just to say, "Hey look, here's a bar we're supposed to jump over, let's set it on the ground and jump over it. Look how great we are!" It's to get a first try at a tech to find out where the real problems lie. Testing something you're already certain will work is a very low-value test.

Again, I bet SpaceX is shooting for that sort of payload, but they might not make it in time. It might be a test that ends up not getting enough analysis done, or isn't finished at all by the time the flight window opens up, or the whole inaugural Red Dragon mission may get pushed back because Red Dragon, that Falcon Heavy, or just SpaceX generally aren't ready.

But I doubt SpaceX would shoot for a test that doesn't try to address the water issue. (Unless they put that greenhouse on Mars, or some other feel-good demo, on Mars.)

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By the way, on Earth, it's very rare for any land to be below sea level, but about half of Mars is below "datum," and some parts of Mars (Hellas Basin) actually have about twice the pressure as datum... so I don't really think it's fair to pick "datum" as analogous to "sea level"... I think early landing sites and early habitation will be more like -5km, which has almost 60% higher pressure/density than datum.

While it's probable they'll aim for below datum (there's several advantages), if you're really optimistic about terraforming you might not want to aim *too* far below datum Just so long as your colony can move uphill faster than your terraforming you should be fine though, so in reality it probably won't matter.

Certainly. But that isn't a reason to rule out SpaceX trying to put on some payload on this initial mission. Water ISRU is a lynchpin of this whole architecture, is low TRL, and so trying it as soon as possible is likely a priority for SpaceX (after demonstrating EDL, of course).

Except the usable-supply-of-water-on-Mars part, that is. This is still a non-trivial problem to solve. Of course, SpaceX might "cheat" and send a few liters of water to Mars along with a sub-scale Sabatier reactor to process some Martian CO2 (and not coincidentally suck all the battery life from the Red Dragon, and then some).

I find most of the approaches to be of the chemical engineering style with very active heating systems. I keep thinking that since heat energy is such a big part of any approach if you don't have a handy nuclear reactor you should be looking at some kind of focused solar system, ideally with natural atmosphere circulation (to eliminate the fan) and some kind of "gill" valves to eliminate mechanical actuators.

Just a thought.

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Certainly. But that isn't a reason to rule out SpaceX trying to put on some payload on this initial mission. Water ISRU is a lynchpin of this whole architecture, is low TRL, and so trying it as soon as possible is likely a priority for SpaceX (after demonstrating EDL, of course).

Except the usable-supply-of-water-on-Mars part, that is. This is still a non-trivial problem to solve. ...

Water is, in fact, ubiquitous on Mars. It exists over the whole planet in the atmosphere and in the regolith. In levels high enough that, with enough work, you can get usable amounts of it. And this is exactly the sort of thing you'd want to demo as soon as possible.

Do the math on the power consumption on the Sabatier and associated hardware (collecting CO2, collecting usable quantities of pure-enough water, electrolysis, storage of O2 and H2 ... ) and then tell me on how it fit into a power-constrained Red Dragon mission at all.

Anytime I see a process that involves a phase change I think "Oh gosh" (or something similar). Heat capacities are in Joules/Kg for 1K temperature rises but with water it's Mj/Kg for no change of temperature. Unless you can get that back through some clever kind of clever "process intensification" design you're going to need a lot of power.

But I think the joker in the pack is that the water being harvested is not pure neutral pH tap water it'll have (depending on source) various assorted chemical species in it that can over time clog filters and corrode hardware. Ever seen a kettle's insides in a "hard" water area? Or what it does to the heater in a washing machine?

Building this kit is easy if you can assume a) Large power source b)Servicing person/robot who can change parts.

BFS. The worlds first Methane fueled FFORSC engined CFRP structured A380 sized aerospaceplane tail sitter capable of flying in Earth and Mars atmospheres. BFR. The worlds biggest Methane fueled FFORSC engined CFRP structured booster for BFS. First flight to Mars by end of 2022. Forward looking statements. T&C apply. Believe no one. Run your own numbers. So, you are going to Mars to start a better life? Picture it in your mind. Now say what it is out loud.

As to retropropulsion, the issue is an energy management program that handles turbulent hypersonic drag down to transonic low altitude transition. Part of this involves lift/drag/props trade-offs to hold the RD in "sweet spots" in the descent for max drag. The desire is to achieve precision landing and economic props utilization together.

As to EI - there are tons more ions than in Earths atmosphere at the same density, because ions penetrate deeper due to the weak magnetosphere (omitting peculiarities involving crustal fields). This can be used in interesting ways.

As to water for ISRU, the problem is in an effective means to supply water to a processing system, both in terms of payload mass, field of action (volume/surface/depth/precipitation), and energy to support. Solve these in an acceptable scale to metric tons and you've got a beginning.

At this point in time, if you have a successful RD landing in an desirable area for a follow on landing, and you even make an ounce of GOX, that would be a triumph. Which could be built upon with the next RD capturing a picture of the other RD across from it, making a liter on the next opposition.

There will be residual hypergolic propellants from the SuperDracos after landing.

Anyone who's seen The Martian knows (j/k) how easy it is to get water from these. But, more realistically, water is one of the combustion products of MMH + NTO, and produces waste heat if that's useful elsewhere in the test procedure.

Downside - water will likely be contaminated by nastier nitrogen based products of partial combustion.

Anything that does not derive water the same way a later full scale architecture would, is not worth any complexity. If they really want to do anything that involves water they can bring a liter or two from earth.

Anything that does not derive water the same way a later full scale architecture would, is not worth any complexity. If they really want to do anything that involves water they can bring a liter or two from earth.

Anything that does not derive water the same way a later full scale architecture would, is not worth any complexity. If they really want to do anything that involves water they can bring a liter or two from earth.

I read your post and to me it does not to seem like it says the same. Maybe I misinterpret it. I put the emphasis on it not being the same as producing the water locally. Especially using similar methods as for mass production later. Producing water from hypergolic propellant was, what I replied to. And that falls under the same argument. It is some kind of local water production but not at all similar to digging it out of the ground. Even extracting the water from the atmosphere is not helpful in that respect as it cannot yield the amounts of water for fuel ISRU and supplying a base later.

Or short and most clearly. Only producing the water in the same way that will be needed for ISRU later is going to give really useful data. Everything else is more PR.

Producing water from the air is similar to later methods (getting it from regolith or dirty ice) & is simple enough to put on an early mission.

Heck, a small scoop that gets some regolith and extracts some water from it also would be even more similar. Not too different than some of the instruments on Viking or MSL, but simpler.

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Chris Whoever loves correction loves knowledge, but he who hates reproof is stupid.

To the maximum extent practicable, the Federal Government shall plan missions to accommodate the space transportation services capabilities of United States commercial providers. US law http://goo.gl/YZYNt0

Producing water from the air is similar to later methods (getting it from regolith or dirty ice) & is simple enough to put on an early mission.

Heck, a small scoop that gets some regolith and extracts some water from it also would be even more similar. Not too different than some of the instruments on Viking or MSL, but simpler.

Which again isn't at all useful for anything more than a stunt, since you'll obtain minute quantities of water from whatever a small scoop can gather (assuming SpaceX would even try to engineer such a thing for a Red Dragon demo mission). From that minute few droplets of water, you'll get ... how much methane exactly? Silly waste of resources to devote that much power, mass, and software development for such a useless activity.

Not a stunt, a demonstration, a proof of concept. Make it work, measure results, feed back into planning for the next time.

We can do that on Earth in a lab or an environment chamber filled with CO2 and a floor covered in iron-rich soil, perchlorate salts and a tiny bit of water. In fact, it HAS been done that way.

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What would you prefer in its place?

The mass/power/data required for an ISRU stunt to be devoted to longer surface life, more/higher fidelity data recording from the powered EDL phases of flight, longer surface life, or basically anything else.

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It's not useful to just say no to something ...

When one has engineering experience and knowledge relevant to the discussion and knows the relatively tiny payoff for a stunt compared to how much effort and spacecraft resources necessary to carry it out, it certainly *IS* useful.